The present invention is directed to a method of hydraulically deforming a pipe-shaped hollow member in a die formed of two die halves. The hollow member is inserted into the die cavity and the two halves of the die are closed for effecting a first deformation of the hollow member. In the first deformation only portions of the outside surface of the hollow member ar displaced into contact with the die cavity surfaces. Subsequently, the open ends of the hollow member are sealed and by applying hydraulic pressure within the hollow member it is deformed into complete contact with the cavity surfaces.
In addition, the present invention is directed to an apparatus for hydraulically deforming the pipe-shaped hollow member where the die is made up of two die halves displaceable toward one another for deforming the hollow member. Means are provided for sealing the open ends of the hollow member. Other means apply hydraulic pressure to the interior of the hollow member.
Such a method or apparatus are part of the state of the art as disclosed in EP 0 294 034 A2. In this known art, the hollow member is placed within an open die and then the ends of the hollow member are closed or sealed. With the die still open a pressure in introduced into the interior of the hollow member by a hydraulic fluid for at least overcoming the frictional forces which the die halves exert on the hollow member when the die is closed and which forces tend to press portions of the wall of the hollow member into the gap between the die halves. The hydraulic pressure within the hollow member is lower than the elastic limit or yield strength of the material of the hollow member.
After applying the internal pressure, the die is closed by moving the die halves toward one another, whereby the cross-section of the hollow member is approximately adapted to the shape of the die cavity. Next, the hydraulic pressure within the hollow member is increased. As a result, the wall segments of the hollow member are stretched beyond the elastic limit until they are completely pressed into contact with the die cavity.
A disadvantage of this known method and apparatus is that the hydraulic pressure increases in the closed interior of the hollow member, due to the volume reduction when the die halves are closed. There is the danger that the hollow member can crack of even burst.
It would be possible to continuously monitor the hydraulic pressure within the hollow member when the die is closed and to lower it to correspond to the reduction of the inside space volume, however, such a procedure can be performed only at not justifiably high costs, because of the control and/or regulation or technology of the installation.
Another, drawback is that the hollow member is filled with a hydraulic fluid prior to the initial deformation. Consequently, it is necessary to provide a perfect seal for the hollow member after it has been filled. This step requires extraordinarily high cost, since the seal must be maintained in a perfect state during the initial deformation as well as the final deformation.